Lithium titanate and method of forming the same

ABSTRACT

A lithium titanate is formed by mixing lithium carbonate powder or lithium hydroxide powder with titanium oxide followed by preparing a mixed slurry of titanium compound powder and a solution containing lithium, followed by depositing a lithium compound by spray-drying.

RELATED APPLICATIONS

This patent application is a continuation-in-part application of a non-provisional application Ser. No. 11/462,520 filed on Aug. 4, 2006 and also claims priority to a provisional patent application Ser. No. 60/917,721 filed on May 14, 2007 and incorporated herewith by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a lithium titanate and a method of forming the lithium titanate. More specifically, the present invention relates to a lithium titanate that has excellent electronic conductivity and excellent electric power-generating capacity.

2. Description of the Related Art

Motor vehicles such as, for example, hybrid vehicles use multiple propulsion systems to provide motive power. The most common hybrid vehicles are gasoline-electric hybrid vehicles, which include both an internal-combustion engine (ICE) and an electric motor. The gasoline-electric hybrid vehicles use gasoline to power the ICE, and an electric battery to power the electric motor. The gasoline-electric hybrid vehicles recharge their batteries by capturing kinetic energy. The kinetic energy may be provided via regenerative braking or, when cruising or idling, from the output of the ICE. This contrasts with pure electric vehicles, which use batteries charged by an external source such as a power grid or a range extending trailer.

The batteries include rechargeable lithium-based cells that typically comprise two dissimilar electrodes, i.e., an anode and a cathode, that are immersed in an ion conducting electrolyte, with a separator positioned between the two electrodes. Electrical energy is produced in the cells by an electrochemical reaction that occurs between the two dissimilar electrodes.

The largest demand placed on the battery occurs when it must supply current to operate the electric motor during acceleration, especially during start-up of the electric motor. The amperage requirements of the electric motor may be over several hundred amps. Most types of batteries that are capable of supplying the necessary amperage have a large volume or require bulky packaging, which results in excessive weight of the batteries and adds cost to the batteries. At the same time, such high currents are only required for short periods of time, usually seconds. Therefore, so called “high-rate” batteries, which provide high currents for short periods of time, are typically ideal for hybrid and pure electric vehicle applications.

Rechargeable batteries that include rechargeable lithium-based cells, which may be characterized as either lithium cells, lithium ion cells, or lithium polymer cells, combine high electric power-generating capacity with the potential for power and cycle-life needed to enable the hybrid vehicles to meet performance standards while remaining economical. By “high electric power-generating capacity”, it is meant that the rechargeable batteries have four times the energy density of lead-acid batteries and two to three times the energy density of nickel-cadmium and nickel-metal hydride batteries. Rechargeable batteries including the lithium-based cells also have the potential to be one of the lowest-cost battery systems.

Lithium titanate represented by the formula Li₄Ti₅O₁₂ (or Li_(4/3)Ti_(5/3)O₄) is considered to be one of the most prospective materials for use in the anodes of rechargeable lithium ion and lithium polymer cells. Lithium titanate, Li₄Ti₅O₁₂, is known from A. Deschanvers et al. (Mater. Res. Bull., v. 6, 1971, p. 699). As it was later published by K. M. Colbow et al. (J. of Power Sources, v. 26, N. 3/4, May 16, 1989, pp. 397-402), Li₄Ti₅O₁₂ is able to act in a reversible electrochemical reaction, while elemental lithium is incapable of such reversible reactions. After detailed research conducted by T. Ozhuku et al. (J. of Electrochemical Society, v. 142, N. 5, 1995, pp. 1431-1435) the Li₄Ti₅O₁₂ started to become considered for use as an anode material for rocking-chair type lithium cells. In fact, U.S. Pat. No. 5,545,468 to Koshiba et al. discloses the use of a lithium titanate having varying ratios of lithium to titanium in the lithium titanate. More specifically, the lithium titanate of the '468 patent is of the formula Li_(x)Ti_(y)O₄, wherein 0.8≦x≦1.4 and 1.6≦y≦2.2, in a cathode for a lithium cell. The '468 patent specifies that fundamentally, x+y≈3. In other words, the '468 patent teaches that the lithium titanate may include different ratios of lithium to titanium, so long as the amount of lithium and titanium together about 3 such that there is a stoichiometric amount of lithium and titanium to oxygen. United States Patent Publication No. 2002/0197532 to Thackeray et al. also discloses a lithium titanate that is used as an anode in a lithium cell. The lithium titanate may be a stoichiometric or defect spinel, in which the distribution of lithium can vary from compound to compound. In addition to an ability to act in the reversible electrochemical reaction, Li₄Ti₅O₁₂ also has other advantages that make it useful in rechargeable lithium-based cells. For example, due to a unique low volume change of the lithium titanate during the charge and discharge processes, the lithium titanate has excellent cycleability, i.e., many cycles of charging and discharging may occur without deterioration of the cells. The excellent cycleability of the lithium titanate is primarily due to a cubic spinel structure of Li₄Ti₅O₁₂. According to data of S. Schamer et al. (J. of Electrochemical Society, v. 146, N. 3, 1999, pp. 857-861) a lattice parameter of the cubic spinel structure (cubic, Sp. gr. Fd-3m (227)) varies from 8.3595 to 8.3538 Å for extreme states during charging and discharging. This linear parameter change is equal to a volume change of about 0.2%. Li₄Ti₁₅O₁₂ has an electrochemical potential versus elemental lithium of about 1.55 V and can be intercalated with lithium to produce an intercalated lithium titanate represented by the formula Li₇Ti₅O₁₂, which has a theoretical electric power-generating capacity of up to and including 175 mA*hrs/g.

Another advantage of Li₄Ti₅O₁₂ is that it has a flat discharge curve. More specifically, the charge and discharge processes of Li₄Ti₅O₁₂ take place in a two-phase system. Li₄Ti₅O₁₂ has a spinel structure and, during charging, transforms into Li₇Ti₅O₁₂, which has an ordered rock-salt type structure. As a result, electric potential during the charge and discharge processes is determined by electrochemical equilibrium of the Li₄Ti₅O₁₂/Li₇Ti₅O₁₂ pair, and is not dependant on lithium concentration. This is in contrast to the discharge curve of most other electrode materials for lithium power sources, which maintain their structure during the charge and discharge processes. For example, although a transition of a charged phase in most cathode materials such as LiCoO₂ is pre-determined, there is still an extended limit of variable composition Li_(x)CoO₂ between these structures. As a result, electrical potential of materials such as LiCoO₂ depends on a lithium concentration in the LiCoO₂, i.e., a state of charge or discharge. Thus, a discharge curve in materials in which the electrical potential is dependent on the lithium concentration in the material is typically inclined and is often a step-like curve.

There is a general consensus within the art that maintenance of excellent electric power-generating capacity correlates to excellent electronic conductivity. Li₄Ti₅O₁₂ includes titanium in a highest oxidation degree of +4, which correlates to very low electronic conductivity. An electronic conductivity of similar compounds is so low that many of those compounds are borderline dielectrics or insulators. As such, power generating capacity of Li₄Ti₅O₁₂ is less than ideal. The same holds true for the lithium titanates of the '468 patent and the '532 publication, as set forth above.

Typically, electronic conductivity of the Li₄Ti₅O₁₂ is improved by doping the Li₄Ti₅O₁₂ with 3d-elements, as disclosed by M. Nakayama et al (Solid State Ionics, v. 117, I. 3-4, 2 Feb. 1999, pp. 265-271). For example, electronic conductivity of Li[Li_((1-x)/3)Cr_(x)Ti_((5-2x)/3)]O₄, which is considered to be a solid solution between Li₄Ti₅O₁₂ and LiCrTiO₄, is better than electronic conductivity of the Li₄Ti₅O₁₂. However, an increase in the amount of Cr ions substituted for titanium ions in the Li₄Ti₅O₁₂ also decreases reversible electric power-generating capacity, as compared to Li₄Ti₅O₁₂, due to electrochemical inactivity attributable to the presence of the Cr ions. The presence of the Cr ions lowers area specific impedance (ASI) and increases rate capability, as compared to ASI and rate capability of Li₄Ti₅O₁₂. The loss in capacity is substantially equal to the share of replaced titanium.

Other attempts to replace the titanium in lithium titanates exhibit similar drawbacks. For example, substitution of titanium in Li₄Ti₅O₁₂ with vanadium, manganese, and iron results in significant loss of reversible electric power-generating capacity during a first charge-discharge cycle. See P. Kubiak, A. Garsia, M. Womes, L. Aldon, J. Olivier-Fourcade, P.-E. Lippens, J.-C. Jumas “Phase transition in the spinel Li₄Ti₅O₁₂ induced by lithium insertion. Influence of the substitution Ti/V, Ti/Mn, Ti/Fe” (J. of Power Sources, v. 119-121, Jun. 1, 2003, pp. 626-630).

In view of the foregoing, there remains an opportunity to provide a lithium titanate that is modified to exhibit excellent electronic conductivity while maintaining reversible electric power-generating capacity that is characteristic of lithium titanate. There is also an opportunity to provide a lithium-based cell that includes the lithium titanate.

SUMMARY OF THE INVENTION AND ADVANTAGES

The subject invention provides a method for synthesizing lithium titanate that include 1) mixing lithium carbonate powder or lithium hydroxide powder with titanium oxide by various mixing methods and using the same as the precursor for calcination, 2) using a synthetic product or mixture containing titanium and lithium obtained from solutions containing both titanium and lithium ingredients as the precursor for calcination and 3) preparing a mixed slurry of titanium compound powder and a solution containing lithium, followed by depositing a lithium compound by spray-drying, to produce a mixture of titanium compound powder and lithium compound and using the same as the precursor for calcination. Each method utilizes production parameters to produce lithium titanate with the quality that satisfies the performance required of secondary batteries.

As such, the lithium titanate of the present invention is suitable for lithium-based cells that are used in rechargeable batteries that are a power source for electric motors in gasoline-electric hybrid vehicles, and the lithium titanate of the present invention materially contributes to the conservation of energy resources by improving performance of the lithium-based cells.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages of the present invention will be readily appreciated, as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 is a schematic view of a rechargeable battery including lithium-based cells;

FIG. 2 is a schematic view of a vehicle including the rechargeable battery of FIG. 2;

FIG. 3 is a lithium titanate composition-valence diagram showing a relationship between ratios of lithium to titanium in lithium titanate to a valence of titanium in the lithium titanate, with diamonds indicating spinel structures, squares indicating non-spinel structures, with filled symbols indicating lithium titanates that include intercalated lithium ions;

FIG. 4 is an X-ray diffraction spectra for conventional Li₄Ti₅O₁₂ of the prior art, synthesized according to Comp. Example 1 in Table 2;

FIG. 5 is an X-ray diffraction spectra for Li₄Ti₅O_(11.985) of the present invention, synthesized according to Example 2 in Table 1;

FIG. 6 is a graph showing a dependence of log(σ) vs. 1/T measured for Li₄Ti₅O_(11.985) of the present invention, synthesized according to Example 2 in Table 1 and measured by the 4-probe method;

FIG. 7 is a kinetic curve of a sintering step whereby Li₄Ti₅O₁₂ is reduced by a H₂/Argon gas mixture (4.81 vol. % H₂), representing dependence of concentration of H₂ on temperature during heating with constant temperature increase of 2.5° C./min.;

FIG. 8 is a kinetic curve of the sintering step of FIG. 7 in log(x) vs. 1/T coordinates, wherein x is x in Li₄Ti₅O_(12-x);

FIG. 9 is a graph showing a dependence of electric power generating capacity (mAh) vs. a number of cycles for a cell including an electrode with Li₄Ti₅O_(12-x) of the present invention wherein a counter electrode is lithium metal;

FIG. 10 is a graph showing a first discharge of a cell including an electrode with Li₄Ti₅O_(12-x) of the present invention wherein a counter electrode is lithium metal;

FIG. 11 is a graph showing a second charge of the cell including the electrode with Li₄Ti₅O_(12-x) of the present invention wherein the counter electrode is lithium metal;

FIG. 12 is a graph showing a 382^(nd) discharge of the cell including the electrode with Li₄Ti₅O_(12-x) of the present invention wherein the counter electrode is lithium metal; and

FIG. 13 is a graph showing a 382^(nd) charge of the cell including the electrode with Li₄Ti₅O_(12-x) of the present invention wherein the counter electrode is lithium metal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A lithium titanate of the present invention is useful in lithium-based cells. The lithium-based cells including the lithium titanate of the present invention are useful in many applications, but are particularly useful in rechargeable batteries for vehicles 10, such as hybrid or electric vehicles 10; however, it is to be appreciated that the lithium-based cells may be used in non-rechargeable batteries. The rechargeable batteries are a power source 12 for an electric motor of the vehicles 10.

The lithium-based cells include an electrolyte, an anode, and a cathode. Electrolytes for the lithium-based cells are typically non-aqueous lithium ion-conducting electrolytes and are known in the art. At least one of the anode and the cathode includes the lithium titanate of the present invention. For example, the lithium-based cell may be further defined as a lithium cell, wherein the cathode comprises the lithium titanate of the present invention. The lithium titanate is typically present in the cathode in an amount of at least 80 parts by weight, more typically from 80 to 90 parts by weight, most typically in an amount of about 82 parts by weight based on the total weight of the cathode. In addition to the lithium titanate, the cathode in the lithium cell also typically includes a conductive agent such as carbon black along with a binder agent, such as polyvinylidene fluoride, which make up the balance of the cathode. More specifically, the carbon black is typically present in an amount of from 8 to 10 parts by weight, more typically about 8 parts by weight based on the total weight of the cathode, and the binder agent is typically present in an amount of from 8 to 12 parts by weight, more typically about 10 parts by weight, based on the total weight of the cathode. The anode in the lithium cells is typically a lithium metal or lithium alloy with magnesium or aluminum.

Alternatively, the lithium-based cell may be further defined as one of a lithium ion cell and a lithium polymer cell, wherein the anode comprises the lithium titanate of the present invention in the amounts set forth above.

When used in rechargeable batteries for hybrid or electric vehicles 10, the cells are typically used in a battery pack, represented by 14 in FIGS. 1 and 2. The battery packs 14 typically include four rows of the cells that are interconnected and extend along each row in overlapping relationship. Each row typically includes five stacks of the cells. However, it is to be appreciated that other configurations of the cells within the battery pack 14 may also be used.

As known in the art, the rechargeable batteries typically include a plurality of the battery packs 14 connected in a circuit in order to provide sufficient energy for powering the vehicle 10. As shown in FIGS. 1 and 2, the circuit is configured with switches 18 and a battery management system 20 disposed in the circuit 16. The battery management system 20 includes a switch control and interface circuit 22 to control energy usage from and recharge of the cells in the battery packs 14.

The lithium titanate of the present invention has the following formula: Li₄Ti₅O_(12-x) wherein x is greater than 0. Typically, 0<x<0.02. In other words, the lithium titanate of the present invention is deficient of oxygen, which has excellent electronic conductivity, as compared to lithium titanate of the above formula that is not deficient of oxygen. At the same time, concentration of lithium in the lithium titanate of the present invention remains the same as for lithium titanate that is not deficient of oxygen. As a result, expected reversible electric power-generating capacity of the lithium titanate of the present invention will remain the same as the reversible electric power-generating capacity of lithium titanate that includes a stoichiometric amount of oxygen.

The effect on electronic conductivity as a result of the oxygen deficiency is attributable to changes in an oxidation state, i.e., valence, of the titanium in the lithium titanate. More specifically, lithium titanates that include titanium atoms in a +3 oxidation state exhibit high electronic conductivity that is characteristic of metal-like material, while lithium titanates that include titanium atoms in a +4 oxidation state exhibit low electronic conductivity that is characteristic of a dielectric material. Referring to FIG. 3, the oxidation state of various lithium titanates is represented on the vertical axis as v(Ti), i.e., valence of titanium. As such, FIG. 3 indicates the relative electronic conductivity of the various lithium titanates, at various states of intercalation, with higher v(Ti) correlating to lower electronic conductivity. Li₄Ti₅O₁₂ is an example of lithium titanate having the titanium atoms in the +4 oxidation state.

During electrochemical intercalation or charging of conventional Li₄Ti₅O₁₂, phase transition from spinel to “rock salt”-type occurs wherein three lithium atoms are intercalated into the conventional Li₄Ti₅O₁₂ to produce Li₇Ti₅O₁₂. Li₇Ti₅O₁₂ has a higher electronic conductivity than the conventional Li₄Ti₅O₁₂ due to the transformation of titanium atoms in the conventional Li₄Ti₅O₁₂ from the +4 oxidation state to the +3 oxidation state during intercalation, as shown in FIG. 3 and as represented by the following equation: Li₄Ti₅O₁₂+zLi⁺+ze⁻→(1−z/3)Li₄Ti⁴⁺ ₅O₁₂+z/3Li₇Ti⁴⁺ ₂Ti³⁺ ₃O₁₂ wherein z represents the number of lithium atoms that are intercalated into the Li₄Ti₅O₁₂. As such, the conventional Li₄Ti₅O₁₂ exhibits variable electronic conductivity based on the state of intercalation and zones of low and high electronic conductivity may exist during intercalation and discharge due to the disparate differences in electronic conductivity between the conventional Li₄Ti₅O₁₂ and Li₇Ti₅O₁₂. Poor electronic conductivity of the conventional Li₄Ti₅O₁₂ causes initial “training” of the cells by low current as well as prevention of a complete charge. These circumstances extremely limit opportunities of use of the conventional Li₄Ti₅O₁₂ for high rate applications.

In accordance with the present invention, it was surprisingly found that the following relationship exists: Li₄Ti₅O₁₂+δH₂→Li₄Ti⁴⁺ _(5-2δ)Ti³⁺ _(2δ)O_(12-δ)+δH₂O↑ In effect, reduction of Li₄Ti₅O₁₂ to form the Li₄Ti₅O_(12-x) results in the transformation of titanium atoms in the Li₄Ti₅O₁₂ from the +4 oxidation state to the +3 oxidation state as a result of charge compensation, thereby exhibiting increased electronic conductivity of the Li₄Ti₅O_(12-x) while retaining the same number of lithium and titanium atoms in the lithium titanate. Stated differently, an average valence of titanium in the lithium titanate of the present invention is less than 4. The practical result of the above finding is that the lithium titanate will exhibit less drastic changes in electronic conductivity at all stages of charge and discharge processes, as opposed to the conventional Li₄Ti₅O₁₂ that exhibits electronic conductivity that is near that of dielectric materials prior to charging, such that different zones of Li₄Ti₅O_(12-x) and Li₇Ti₅O_(12-x) will exhibit more uniform media for charge and discharge processes, as compared to conventional Li₄Ti₅O₁₂, which is advantageous for high-rate applications.

Since the same numbers of electrochemically active lithium and titanium atoms are present as are present in the Li₄Ti₅O₁₂, expected reversible electric power-generating capacity will be the same for the Li₄Ti₅O_(12-x) as for the Li₄Ti₅O₁₂. The Li₄Ti₅O_(12-x) also retains the same spinel structure as Li₄Ti₅O₁₂, which has excellent cycleability. As set forth above, typically, 0≦x≦0.02 in order to maintain the lithium titanate having the same spinel structure as the Li₄Ti₅O₁₂. More specifically, referring to FIG. 3, the lithium titanate of the present invention, by having the oxygen deficiency, shifts the Li₄Ti₅O_(12-x) to a position represented by “A” in FIG. 3 due to the transformation of titanium atoms in the Li₄Ti₅O₁₂ from the +4 oxidation state to the +3 oxidation state, with the position represented by “B” indicating an intercalation state of the Li₄Ti₅O_(12-x). The value of x, in order to maintain the same spinel structure as Li4Ti5O12, is limited since lithium titanate with structure of Li2Ti3O7 will form if the amount of titanium in the +3 oxidation state becomes too high. Li₂Ti₃O₇ has an orthorhombic crystal structure with space group Pbnm (62). Although Li₂Ti₃O₇ may be suitable for certain applications, the spinel structure of Li₄Ti₅O₁₂ is preferred due to the ability to intercalate more lithium into the structure than can be intercalated into the Li₂Ti₃O₇ and also due to the fact that Li₄Ti₅O₁₂ exhibits low volume change of from 8.3595 to 8.3538 Å between intercalated and deintercalated states, which provides the excellent cycleability.

A method of forming the Li₄Ti₅O_(12-x) includes the step of providing a mixture of titanium dioxide and a lithium-based component. Titanium dioxide can be used both in the form of rutile and in the form of anatase, as well as any form of titanium oxide-hydroxide (such as Ti(OH)_(2x)O_(2-x)). Any lithium-based component that is typically used for forming Li₄Ti₅O₁₂ may be used. Typically, the lithium-based component is selected from the group of lithium carbonate, lithium hydroxide, lithium oxide, and combinations thereof, and the lithium-based component is typically at least 99% pure. Lithium salts or organic acids can also be used. Typically, the lithium-based component and titanium oxide are present in the mixture in amounts necessary to ensure an atomic ratio Li/Ti=0.8 in the final lithium titanate of the present invention.

The mixture including the titanium dioxide and the lithium-based component is sintered in a gaseous atmosphere comprising a reducing agent to form the lithium titanate. More specifically, the mixture is sintered at a temperature of at least 450° C., more typically from about 500 to 925° C., most typically from about 700 to about 920° C., for a period of at least 30 minutes, more typically from about 60 to about 180 minutes.

The reducing agent may be any agent that is capable of reducing the oxygen in the Li₄Ti₅O₁₂ and is typically selected from the group of hydrogen, a hydrocarbon, carbon monoxide and combinations thereof. The reducing agent is typically present in the gaseous atmosphere in a concentration of at least 0.1% by volume, more typically from about 1 to about 100% by volume, in order to sufficiently reduce the Li₄Ti₅O₁₂ to form the Li₄Ti₅O_(12-x).

In addition to the reducing agent, the gaseous atmosphere typically includes another gas selected from the group of an inert, an inactive gas, and combinations thereof. Any inert gas may be used, such as any noble gas, in order to prevent unwanted side reactions during sintering and in order to prevent introduction of impurities into the Li₄Ti₅O_(12-x). Inactive gas that may be used is, for example, pure nitrogen.

The following examples are meant to illustrate the present invention and are not to be view in any way as limiting to the scope of the invention.

EXAMPLES

Lithium titanate of the present invention having the formula Li₄Ti₅O_(12-x) is formed according to the method of the invention as set forth above. More specifically, conventional Li₄Ti₅O₁₂ is first formed by forming a mixture including titanium dioxide and a lithium-based compound. The mixture is formed by introducing the titanium dioxide and the lithium-based compound into a vessel in the amounts shown in Table 1. The titanium dioxide and the lithium-based compound are mixed and milled in a ball mill for a period of about 60 minutes at least 150 rpm rotation speed using a particle size distribution measurement till particle size less than 5 mkm, more preferably less than 2 mkm, with unimodal distribution to ensure sufficient mixing of the titanium dioxide and the lithium-based component. The mixture is then sintered in a gaseous atmosphere, created by a gas or gas mixture with constant flow at temperatures and times as indicated in Table 1. The gas or gas mixture includes a reducing agent and an inert gas or inactive gas in the amounts indicated in Table 1. The resulting lithium titanate has the formula Li₄Ti₅O_(12-x) with 0<x<0.02. Relevant properties of the lithium titanate of the present invention are also included in Table 1 below.

TABLE 1 Component Example 1 Example 2 Example 3 Mixture Titanium Dioxide, pbw 72.992 80.655 76.632 based on total weight of mixture Lithium-based 27.008 — 14.178 Component A, pbw based on total weight of mixture Lithium-based — 19.345 9.190 Component B, pbw based on total weight of mixture Total 100.0 100.0 100.0 Gaseous Reducing Agent A flow, 0.002 — — Atmosphere L/(min · kg) based on total weight of mixture Reducing Agent B flow, — 0.0025 — L/(min · kg) based on total weight of mixture Reducing Agent C flow, — — 0.05 L/(min · kg) based on total weight of mixture Inert Gas A flow, 0.048 0.0225 — L/(min · kg) based on total weight of mixture Inactive Gas B flow, — — 0.095 L/(min · kg) based on total weight of mixture Total 0.05 0.025 0.1 Sintering Time, min 120 100 180 Sintering Temperature, 850 900 800 ° C. X value in Li₄Ti₅O_(12-x) 0.009 ± 0.001 0.015 ± 0.001 0.005 ± 0.001 Reversible Electric 168 170 160 Power-Generating Capacity, mA*hrs/g Crystal Structure 8.36012 8.35978 8.36023 Parameter (a), Å, at 300 K Logarithm of DC −5.2 −4.7 −5.9 Electronic Conductivity, (S cm⁻¹), at 300 K Lithium-based Component A is Li₂CO₃. Lithium-based Component B is LiOH. Reducing Agent A is H₂. Reducing Agent B is CH₄ (methane). Reducing Agent C is CO (carbon monoxide). Inert Gas A is Argon. Inactive Gas B is N₂ (nitrogen).

COMPARATIVE EXAMPLE

Conventional lithium titanate having the formula Li₄Ti₅O₁₂ is formed in the same manner as set forth above; however the reducing agent is not present in the gaseous atmosphere. The amounts of the components used to form the conventional lithium titanate are shown below in Table 2, along with relevant properties of the conventional lithium titanate.

TABLE 2 Comp. Comp. Component Example 1 Example 2 Mixture Titanium Dioxide, pbw 72.992 80.655 based on total weight of mixture Lithium-based 27.008 — Component A, pbw based on total weight of mixture Lithium-based — 19.345 Component B, pbw based on total weight of mixture Total 100.0 100.0 Gaseous Inert Gas A flow, 0.1 — Atmosphere L/(min · kg) based on total weight of mixture Inactive Gas B flow, — 0.2 L/(min · kg) based on total weight of mixture Total 0.1 0.2 Sintering Time, min 180 120 Sintering Temperature, 850 900 ° C. X value in Li₄Ti₅O_(12-x) 0 ± 0.0005 0 ± 0.0005 formula Reversible Electric 145 150 Power-Generating Capacity, mA*hrs/g Crystal Structure 8.36055 8.35915 Parameter (a), Å, at 300 K Logarithm of DC <−9 ~−9 Electronic Conductivity, (S cm⁻¹), at 300 K

Results

With reference to the reversible electric power-generating capacity and the electronic conductivity of the Examples and the Comparative Example, it is apparent that the lithium titanates of the present invention exhibit higher electronic conductivity than conventional lithium titanates of the Comparative Examples, while exhibiting even higher reversible electric power-generating capacity.

Specifically, XRD spectra are received on an x-ray diffractometer Bruker D4 on CuK_(α) radiation with Sol-X detector. All samples listed in Table 1 and 2 give well-defined spectra correspond to cubic structure (Sp. gr. Fd-3m (227)). Small amounts of residual TiO₂ (<0.5%) are present in most of samples. Using a full-profile analysis method, with conventional structure model (see for example, S. Scharner, W. Wepner, P. Schmid-Beurmann. Evidence of Two-Phase Formation upon Lithium insertion into the Li_(1.33)Ti_(1.67)O₄ Spinel, Journal of the Electrochemical Society. v. 146, I. 3, pp. 857-861, 1999), parameter (a) of a cubic crystal lattice is calculated, and is shown in the Tables 1 and 2. Two typical spectra, one for Li₄Ti₅O₁₂ of the prior art represented by Comp. Examples 1 and 2, and one for Li₄Ti₅O_(11.985) of the present invention represented by Example 2, are presented on FIGS. 4 and 5, respectively.

Electronic conductivity of the Examples is measured on 20 mm diameter, 2-3 mm thick pellets that have been pressed and tempered inside powder samples under synthesis conditions until an equilibrium state is reached. Measurements are made by the 4-probe method on direct current, under potential of 90 volts. Attempts to receive reliable data for Li₄Ti₅O₁₂ samples (Table 2, Comp. Examples 1 and 2) are unsatisfactory, as the conductivity of these samples lies very close to a low limit of measurement for this method. Therefore, only order of conductivity is determinate. Results of measurements for Li₄Ti₅O_(11.985), synthesized according to Example 2 in Table 1, in a narrow temperature interval of about room temperature, are shown on FIG. 6. Main sources of measurement discrepancies are the nature of compacted powder samples with significant porosity, as well as proximity to grain boundaries and contact effects.

The kinetics of the sintering step for reducing the Li₄Ti₅O₁₂ is tested through the Temperature Controlled Reduction method. During linear heating of samples under gaseous atmosphere including the reducing agent, gas concentration is measured after flowing past the sample. Referring to FIG. 7, dependence of concentration of hydrogen, i.e., the reducing agent, against temperature of Li₄Ti₅O₁₂ is shown. A difference between initial concentration of hydrogen and concentration of hydrogen after the gaseous atmosphere flows past the sample gives an amount of hydrogen used for the sintering process. By integration of this curve, using values of sample mass and gas mixture flow, it is possible to calculate the value of x in the formula Li₄Ti₅O_(12-x) as a function of temperature. The reduction during the sintering step becomes appreciable after 450° C. and proceeds smoothly until 925° C. FIG. 8 shows a dependence of the logarithm of x in formula Li₄Ti₅O_(12-x) against reverse absolute temperature (in Kelvin). This curve has an Arrhenius-like character and is close to linear in the temperature interval 500° C.<T<925° C.

FIG. 9 shows that the lithium-based cell that includes the Li₄Ti₅O_(12-x) maintains electric power generating capacity after many cycles, and FIGS. 10-13 illustrate flat charge and discharge curves of the Li₄Ti₅O_(12-x), even after many cycles of charge and discharge.

The quality of the active substance affecting the performance of a lithium secondary battery is obviously regulated by its composition and is also associated with various powder properties including particle size, particle size distribution, specific area, crystallinity, crystal size, crystal size distribution, pore volume, pore volume distribution and crystallinity. For these properties of the active substance, the crystallinity is determined by the heating temperature and heating time when mixing and heating the lithium compound and titanium compound selected, and the powder properties are determined by the subsequent pulverization and sieving.

However, for example, in case improvement of the crystallinity and micronization of the crystal particles are regulated simultaneously, improvement of the crystallinity is achieved by focusing on the heating temperature and heating time by setting a high temperature and long heating time while micronization is generally regulated at low temperature for a short time. The regulating range is thus very narrow because the conflicting operations must be performed simultaneously. Or similarly, for example, when the active substance is produced from secondary particles and in case coarsening of the secondary particles and micronization of the primary particles are regulated simultaneously, the coarsening of the secondary particles is achieved by high temperature and long heating time while the micronization of the primary particles is generally regulated at low temperature for a short time.

Similar to the aforementioned case, the regulating range is thus very narrow because the conflicting operations must be performed simultaneously. Furthermore, the regulating range is even narrower if the aforementioned four items are to be regulated to reach certain targets. Accordingly, the introduction of new production parameters is expected for regulating even higher quality for the active substance for high battery performance.

The methods proposed for synthesizing lithium titanate include mixing lithium carbonate powder or lithium hydroxide powder with titanium oxide by various mixing methods and using the same as the precursor for calcination, using a synthetic product or mixture containing titanium and lithium obtained from solutions containing both titanium and lithium ingredients as the precursor for calcination and preparing a mixed slurry of titanium compound powder and a solution containing lithium, followed by depositing a lithium compound by spray-drying, to produce a mixture of titanium compound powder and lithium compound and using the same as the precursor for calcination.

According to the present invention, multiple lithium compounds are utilized as the starting materials at the same time for thermal synthesis of lithium titanate, in that the thermal synthesis is conducted by focusing on the behavior of the inherent property of each lithium compound and their behaviors in the reaction with the titanium compound and regulating the powder properties including the crystalline conditions such as crystallinity, crystal size and crystal size distribution and powder properties such as powder particle size and specific area that reflect the quality of the lithium titanate, so that the quality can meet the performance required of lithium batteries.

The method of the present invention for producing lithium titanate involves thermal synthesis after mixing two or more lithium compounds with titanium oxide. The atomic transfer accompanying the formation of lithium titanate during the thermal synthesis of lithium titanate is considered to depend on whether it is a solid-solid reaction or liquid-solid reaction. Once lithium titanate is formed, the growth of the crystal particles depends on the existing condition of the surrounding particles, temperature and time.

Alluding to the above, the lithium carbonate melts at 726° C. and decomposes to give lithium oxide and carbon dioxide at temperatures higher than 1500° C. On the other hand, lithium hydroxide melts at 450° C. and decomposes to give lithium oxide at temperatures higher than 924° C. Specifically, the heating process for synthesizing lithium titanate is not the same as in the case of utilizing a mixture of lithium carbonate and titanium oxide and the case of utilizing a mixture of lithium hydroxide and titanium oxide in thermal synthesis. When synthesis by calcination is conducted with titanium oxide at a temperature below the melting point of the respective lithium compound, lithium carbonate and titanium oxide undergo a solid-solid reaction while lithium hydroxide and titanium oxide undergo a liquid-solid reaction, at 500° C. for example. In case of a liquid-solid reaction, the atomic transfer accompanying the formation of lithium titanate is considered to proceed more readily compared to the case of a solid-solid reaction.

Once lithium titanate is formed, the growth of the crystal particles depends on the existing condition of the surrounding particles, temperature and time. The temperature and time vary with the type of starting materials utilized, which expands the quality control range for lithium titanate due to the new parameters. In the present invention, representative compounds of lithium carbonate, lithium hydroxide and titanium oxide are products procurable in the industry, but the lithium compounds and titanium compound are not limited to them.

Also, multiple titanium compounds can be utilized, and pre-prepared titanium and lithium compounds can be utilized as part of the starting material for the thermal synthesis. Application example, include and not limited to titanium oxide powder (−325 mesh) produced by Alfa Aesar, lithium carbonate powder (−325 mesh) produced by FMC and lithium hydroxide powder (−200 mesh) produced by Science Lab were utilized. In addition to 30.5 g titanium oxide, sample 1 containing 11.3 g lithium carbonate, sample 2 containing 8.5 g lithium carbonate and 3.3 g lithium hydroxide, sample 3 containing 5.7 g lithium carbonate and 6.5 g lithium hydroxide, sample 4 containing 2.8 g lithium carbonate and 9.8 g lithium hydroxide and sample 5 containing 13.0 g lithium hydroxide were individually mixed thoroughly in a mixing crucible. The mixtures were fed individually to a ceramic reactor, and thermal synthesis was conducted in a muffle furnace. The heating conditions include heating from room temperature to 150° C. and maintaining the temperature for 2 h and subsequently heating to 450° C. and maintaining the temperature for 2 h and further heating to 950° C. and maintaining the temperature for 8 h, followed by cooling to room temperature to obtain the synthetic products.

Growth of primary particles and an inhibitory effect on the secondary particle size with increasing quantity of starting material lithium hydroxide are verified. Specifically, despite the fact that the same titanium oxide is utilized and the heating condition is the same, better quality can be regulated by increasing the quantity of lithium hydroxide to replace lithium carbonate. Also, differences in crystallinity are verified. The differences in the batteries are evaluated and the performance is verified. These new production parameters are provided by the present invention to give the aforementioned differences in obtaining lithium titanate of desired quality.

The invention has been described in an illustrative manner, and it is to be understood that the terminology which has been used is intended to be in the nature of words of description rather than of limitation. Obviously, many modifications and variations of the present invention are possible in light of the above teachings, and the invention may be practiced otherwise than as specifically described. 

1. A method of forming a lithium titanate having the following formula: Li₄Ti₅O_(12-x) wherein x is greater than 0, said method comprising the steps of: providing a mixture of titanium oxide and a lithium-based component such that the lithium based component and the titanium oxide are present in the provided mixture in amounts necessary to ensure an atomic ratio of lithium to titanium of 0.8; and, sintering the mixture in a gaseous atmosphere comprising a reducing agent to form the lithium titanate.
 2. The method of claim 1 wherein the lithium-based component is selected from the group of lithium carbonate, lithium hydroxide, lithium oxide, and combinations thereof.
 3. The method of claim 1 wherein the reducing agent is selected from the group of hydrogen, a hydrocarbon, carbon monoxide, and combinations thereof.
 4. The method of claim 1 wherein the reducing agent is present in the gaseous atmosphere in a concentration of at least 0.1% by volume.
 5. The method of claim 1 wherein the gaseous atmosphere further comprises another gas selected from the group of an inert gas, an inactive gas, and combinations thereof.
 6. The method of claim 1 wherein the mixture is sintered at a temperature of at least 450.degree. C.
 7. The method of claim 6 wherein the mixture is sintered for a period of at least 30 minutes. 